Mineralogy and geochemistry of ODP Hole 176-735B minerals

We present a synthesis of some 20,504 mineral analyses of ~500 Hole 735B gabbros, including 10,236 new analyses conducted for this paper. These are used to construct a mineral stratigraphy for 1.5-km-deep Hole 735B, the only long section of the lower crust drilled in situ in the oceans. At long wavelengths, generally >200 m, there is a good chemical correlation among the principal silicate phases, consistent with the in situ crystallization of three or four distinct olivine gabbro bodies, representing at least two major cycles of intrusion. Initial cooling and crystallization of these bodies must have been fairly rapid to form a crystal mush, followed by subsequent compaction and migration of late iron-titanium-rich liquids into shear zones and fractures through which they were emplaced to higher levels in the lower crust where they crystallized and reacted with the olivine gabbro host rock to form a wide variety of ferrogabbros. At the wave lengths of the individual intrusions, as represented by the several olivine gabbro sequences, there is a general upward trend of iron and sodium enrichment but a poor correlation between the compositions of the major silicate phases. This, together with a wide range in minor incompatible and compatible element concentrations in olivine and pyroxene at a given Mg#, is consistent with widespread permeable flow of late melt through these intrusions, in contrast to what has been documented for a 600-m section of reputedly fast-spreading ocean crust in the Oman Ophiolite. This unexpected finding could be related to enhanced compaction and deformation-controlled late-stage melt migration at the scale of intrusion at a slow-spreading ocean ridge, compared to the relatively static environment in the lower crust at fast-spreading ridges.

本研究整合了约20504件735B钻孔（Hole 735B）中约500件辉长岩（gabbro）样品的矿物分析（mineral analysis）数据，其中包含为本论文新完成的10236件分析数据。这些数据被用于构建深度达1.5千米的735B钻孔的矿物地层序列（mineral stratigraphy）——该钻孔是全球唯一一处在洋域内原位（in situ）钻取的下地壳（lower crust）长剖面。在长波长（通常大于200米）尺度下，主要硅酸盐相（silicate phase）之间呈现良好的化学相关性，这与3至4个独立橄榄辉长岩（olivine gabbro）岩体的原位结晶过程相符，代表了至少两期主要的侵入旋回（intrusion cycle）。这些岩体的初始冷却与结晶过程应相对迅速，形成了晶粥（crystal mush），随后发生压实作用以及富铁钛残余熔体的迁移：熔体沿剪切带（shear zone）与裂隙（fracture）运移至下地壳更高层位，在此结晶并与寄主橄榄辉长岩发生反应，形成多种铁质辉长岩（ferrogabbro）。以各橄榄辉长岩序列为代表的单个侵入体尺度下，整体呈现出铁与钠向上富集的趋势，但主要硅酸盐相的成分之间相关性较弱。这一特征，结合给定镁值（Mg#）下橄榄石（olivine）与辉石（pyroxene）中微量不相容元素（incompatible element）及相容元素（compatible element）浓度的大范围分布，表明晚期熔体在这些侵入体中发生了广泛的渗流（permeable flow）作用——这与阿曼蛇绿岩（Oman Ophiolite）中一段公认的快速扩张洋壳600米剖面的研究结果相悖。与快扩张洋脊（fast-spreading ridge）下地壳相对静态的环境相比，这一意外发现可能与慢扩张洋脊（slow-spreading ocean ridge）侵入体尺度下压实作用增强、变形控制的晚期熔体迁移过程相关。